Methods and apparatuses for obtaining sectional images of an eye by means of a Scheimpflug camera are sufficiently known from the state of the art. Thus, for example from document DE 10 2005 026 371, a so-called Scheimpflug recording device is known, with which, by means of a projection device, an eye is illuminated with a light slit, wherein, by means of a monitoring device, a sectional image that is produced in this way is recorded. The monitoring device is substantially formed from a camera, with which an objective lens and image plane intersects an object plane of the sectional image in a joint point. It is further known to store an image data set which has been recorded in this way and, by means of a digital image analysis, to further process said image data set, for example for establishing optical boundary surfaces.
Optical coherence interferometers for an optical coherence tomography (OCT) are likewise sufficiently known from the state of the art. In the field of ophthalmoscopy, said analysis instruments are regularly utilized for examining, in a detailed manner, an eye in the region of a front eye section, of a rear retinal eye section or are also utilized for a so-called full eye scan. In optical coherence interferometry, with the aid of an interferometer, coherent light is utilized for imaging and for distance measurements at reflective and scattering eye tissue. Due to changes in an index of refraction occurring at optical boundary surfaces of the eye and due to volume scatterings, by means of an optical coherence interferometer, measurable signals can be obtained.
The basic principle of optical coherence interferometry is based on white light interferometry and compares the propagation time of a signal with the aid of an interferometer, such as a Michelson interferometer. Here, an optical reference arm of a known optical path length is taken as a reference for an optical measurement arm or measurement beam with which an eye to be examined is scanned. The interference of the signals from the two arms results in a pattern from which a relative optical path length within a depth profile, which is also called A-scan (amplitude-mode scan), can be read. In multidimensional raster methods, a measurement beam can be guided transversely in one or two directions, which results in a planar tomogram, which is also called B-scan (brightness-mode scan). By means of a depth adjustment of a measurement range, a three-dimensional volume can also be recorded as a so-called C-scan (C-mode scan).
Unlike in conventional light microscopy, in optical coherence tonometry, the transverse resolution is decoupled from the longitudinal resolution. The transverse resolution is determined by the numerical aperture of the optics used. The longitudinal spatial resolution into a depth of the material, in contrast, depends on a spectral width of the light used.
With the coherence tonometry methods used in ophthalmology, two basic types can substantially be distinguished. With a first type, a reference arm of an interferometer can be changed with respect to its length and an intensity of the interference can continuously be measured without a spectrum being taken into account here. This method is called time domain method according to a signal measurement in the time domain. With the second type, for determining the measurement values, the spectrum is taken into account and an interference of the individual spectral components is gathered. This method is called frequency domain method. With the frequency domain method, a movable reference is not needed, whereby a simple and quick simultaneous measurement becomes possible. In particular, a complete piece of information on a depth can be established. In frequency domain OCT, two subgroups are again distinguished, with which, on the one hand, the signal is temporally encoded, which means sequentially recorded, or spatially encoded, which means spatially split, but simultaneously recorded. Since the spectral information which has been obtained by means of the spatial splitting of the signal can be gathered by means of a spectrometer, this method is also called spectral domain OCT.
A method for obtaining an OCT full eye scan is further known, with which an optical coherence interferometer can be combined with a further imaging analysis system. Since, using the optical coherence interferometer, tomographic images of different regions of the eye at different reference arm lengths are scanned, these partial scans have to be assembled to a full image of the eye. Here, the partial scans overlap in order to make an accurately fitting combination of obtained image data sets possible. The other analysis system can be employed for supplementing the full eye scan with further image data or, for example, topographic data of the cornea. Here, the full eye scan represents a reference image data set which, according to the requirements of a required eye examination, is partially supplemented with the image data of the other analysis system in the region of the eye section to be examined. This is advantageous since optical boundary surfaces and, in particular, an eye length can be determined in a particularly accurate way using the optical coherence interferometer in contrast to other analysis systems. Thus, a measurement of an eye length having a comparable accuracy is not possible using a Scheimpflug system.
With the known OCT methods, it is disadvantageous that an image record cannot directly be obtained, i.e. that a corresponding period of time is required for scanning the eye section to be recorded by means of the measurement beam. If a depth scan is to be procured, it is moreover required to adapt the reference arm with the corresponding expenditure of time. With the time domain or the frequency domain method, a sequential recording or a spectral tuning of a light source of the coherence interferometer might be required, which likewise prevents a simultaneous recording of an eye section. Thus, eye movements in the time interval of the image recording using the optical coherence interferometer can lead to a falsification of the measurement results. Due to the convex surface of the cornea of the eye, in the case of an offset of the measurement beam perpendicular to the eye in an X axis, there is likewise a distance change in the direction of a Z axis, and there might be an offset in a Y axis. The movement of the eye consequently causes a change in the curvature of the corneal surface relative to the measurement beam and might cause a change in an index of refraction in the examination region of the eye. In particular due to the change in the curvature of the cornea, another measurement error results since the measurement beam is diverted in a deviating way, for example at the corneal surface, due to the movement of the eye and the change in the curvature which is caused thereby. Although movements of the eye can be identified by an offset of a received signal, this does not make a correction of a measurement error possible which results from a change in the curvature.
When recording an image of an eye section using a Scheimpflug system, the problem of measurement errors as a consequence of eye movements occurs only rarely, since the entire sectional image, in contrast to a scanned image, is recorded substantially simultaneously, since an exposure time of a camera chip is comparatively short. If a comparatively quick eye movement is nevertheless effected within an exposure time, a recorded sectional image appears to be blurry. This can, however, during a measurement regularly be prevented due to the simultaneous image recording which is comparatively quick.